With the higher integration of LSI, it is apparent that conventional monolayer resist systems have already reached the utmost limit of resolution hence a method of forming a high shape ratio having a large layer thickness but a fine pattern has been proposed by employing a multilayer resist system. That is, an organic polymer thick layer is formed as a first layer and a resist thin layer is provided thereon as a second layer, and the second resist material layer is irradiated with high energy radiation to thereby perform development. This method aims at obtaining a pattern having high rectangular property by oxygen plasma etching (O.sub.2 RIE) the first organic polymer layer anisotropically using the positive image obtained above as the mask (see Lin, Solid State Technology, Vol. 24, p. 73 (1981)).
In this case, the second resist material layer should be highly resistive to O.sub.2 RIE hence it is suggested to use a silicon-containing polymer as the second layer.
Further, Bowden et al., reported polytrimethylsilylbutetnyl sulfone (see Society of Photooptical Instrumentation Engineering Abstract, 631-01 p. 14 (1986)). However, as these resist materials are low in a silicon content and silicon is introduced into the side chain, the resistance to oxygen plasma is not sufficient and cannot be used as the mask of the etching of the first organic polymer layer. Further, a non-swelling type resist capable of alkali development is necessary for forming a pattern of high resolution.
Various attempts have been done for the development of resists which have oxygen plasma resistance and alkali development suitability, and they have been partly used for g-line exposure (exposure wavelength of 436 nm) or other exposure.
For example, JP-A-1-283555 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-4-36754, JP-A-4-130324 and JP-A-2-29652 can be exemplified. However, a pattern of high resolution and rectangular shape cannot be obtained by these techniques because light absorption of these resists to exposure light is too large for the formation of fine patterns of a line width of 0.3 .mu.m or less with KrF excimer laser beams, etc.
For reducing light absorption to exposure light, solution to the problem has been attempted variously using a small amount of photoacid generator and a compound whose alkali-insoluble group is decomposed by the generated acid and becomes alkali-soluble.
These techniques are disclosed in JP-A-63-218948, JP-A-63-241542, JP-A-4-245248 and JP-A-6-184311, for instance.
However, when these techniques are applied for the formation of ultrafine patterns of a line width of 0.2 .mu.m or less, a rectangular pattern cannot be obtained because the layer thickness extremely decreases after development as even the unexposed area is partly exposed due to the diffraction of light. Further, dimensional fluctuation becomes large in some resists and mask dimension can hardly be reproduced in the pattern transfer to the lower layer in the next oxygen plasma process as the silicon content in the resist is low.